It is known to prepare methacrylic acid by gas phase oxidation of methacrolein or isobutyraldehyde with oxygen- and steam-containing gas mixtures over molybdenum-and phosphorus-containing catalysts.
Most of the existing processes for preparing methacrylic acid start from methacrolein obtained by catalytic oxidation of isobutylene or tertiary butanol. In the first-second connected process, the reaction gas of the primary oxidation stage where isobutylene or tertiary butanol was converted into methacrolein is fed directly, without isolation of the methacrolein formed, into the secondary oxidation stage where the methacrolein is oxidized to methacrylic acid over Mo- and P-containing heteropolyacids. In the alternative first-second separated process, the methacrolein produced in the primary oxidation stage is separated from the reaction gas, is mixed with O.sub.2 or air, steam and an inert gas and is then fed to the secondary oxidation stage. Other processes start from isobutyraldehyde or from methacrolein obtained by condensation of propanal and formaldehyde in the presence of secondary amines or animals as catalysts. The problem with all the processes mentioned is that
(a) the formation of methacrolein in the primary stages is by the very nature of the reaction accompanied by the formation of high-boiling byproducts which reduce the activity and selectivity of the catalysts of the secondary oxidation stage and by being precipitated in a finely divided form cause an increase in the pressure drop; and
(b) the existing catalysts for the oxidation of methacrolein do not give complete conversion of the methacrolein in the secondary oxidation stage, so that unconverted methacrolein needs to be isolated for economic reasons and recycled into the oxidation stage. The strong tendency of the product mixture obtained in the secondary oxidation stage, containing methacrolein and unsaturated high boilers, to polymerize and to form difficult-to-remove aerosols easily leads to solid deposits in the downstream cooling zones and heat exchangers and hence to an increase in the pressure losses, frequently after short operating periods. In addition, the high boilers can pass as aerosols together with the recycled methacrolein-containing gas streams into the oxidation stage, there to initiate the difficulties mentioned under (a).
Various solutions to these problems have been proposed, but they are not satisfactory and in general are only directed to the two-stage processes based on isobutylene/tertiary butanol.
For the first-second connected process it is proposed for example in U.S. Pat. No. 4,558,028 to use low pressure drop hollow cylinders of defined composition as catalysts in the methacrolein oxidation stage in order to moderate the adverse consequences of the deposition of high-boiling byproducts, such as terephthalic acid and tarlike mixtures. However, this measure does not constitute a fundamental solution to the overall problem, since the adsorption or deposition on the catalyst is not prevented. To suppress the deposition of polymer in the critical cooling zones downstream of the secondary oxidation stage and the blowthrough of aerosols, GB Patent 2,116,963 proposes for the same process that the cooled reaction gas emerging from the oxidation reactor at 250.degree. C. be rapidly cooled down to about 50.degree. C. with an aqueous methacrylic acid solution in a cooling tower equipped with perforated plates without downcomers and that the offgas leaving the cooling zone in the form of an aerosol be washed in a Venturi washer at about 40.degree. C. with an aqueous methacrylic acid solution before being fed into a methacrylic acid absorber. A portion of the methacrolein-containing offgas from the absorber is reacted in a third oxidation reactor, while the remaining portion of the offgas is washed in a water-operated absorber at from 10.degree. to 15.degree. C. to remove the methacrolein. All in all, the process of GB Patent 2,116,963 demands a great deal in terms of apparatus and is associated with losses of useful product of not less than 1 mol %.
In the typical first-second separated processes of GB Patents 2,041,930 and 2,045,759, the methacrolein produced from isobutylene in the primary oxidation stage is washed together with the bulk of the organic byproducts out of the reaction gas in two absorption stages, then in GB Patent 2,041,930 charged as an aqueous solution onto the upper end of a stripping column and driven out there with cycle gas from the secondary oxidation stage at base of column temperatures of 120.degree. C. The cycle gas is charged with methacrolein and water vapor is passed direct to the oxidation reactor. In this process too it cannot be ruled out that high-boiling byproducts having a catalyst-poisoning action pass into the second oxidation stage, since the aqueous methacrolein solution is introduced at the upper end of the stripping column. The reaction gas from the secondary oxidation reactor is, according to British Patent 2,045,759 GB, passed direct or after indirect cooling to 150.degree. C. into a cooling tower and is quenched there with a recycled stream of liquid to about 40.degree. C., while the cooling tower offgas, which contains methacrolein, water and byproducts, is recycled into the oxidation stage. According to GB Patent 2,116,963, the measures of GB Patent 2,045,759 are not suitable for preventing the deposition of polymeric solids in the cooling zones and the formation of aerosols and/or their passing into the oxidation reactor.
EP Patent Application 194,620 describes a process for preparing methacrylic acid by gas phase oxidation of methacrolein which has been obtained by reacting propanal with formaldehyde and which contains interfering and/or high-boiling byproducts comprising inter alia 2-methylpentenal, dimeric methacrolein, 2-methylpentanal, 3-methoxyisobutyraldehyde as well as from 0.5 to 3% by weight of water and small amounts of methanol, formaldehyde and propionaldehyde. A recommendation is to purify the methacrolein before introduction to the oxidation reactor by absorption or distillation in such a way that the concentration of oxygen-containing and/or unsaturated compounds having more than 4 carbon atoms in the gas mixture is less than 0.2% by weight, based on the methacrolein. Otherwise this European patent application reveals no measures for preventing solid deposits in the critical cooling zones and aerosol formation in the isolation of methacrylic acid and of uncoverted methacrolein which go beyond the above-discussed proposals.
In the course of further work on the last process it was found that prepurification of the methacrolein by the method of EP Patent Application 194,620 is not completely sufficient to prevent, in sustained industrial operation, temporary or irreversible poisoning of the oxidation catalysts by impurities in the methacrolein on using the prior art measures for isolating the methacrylic acid and the unconverted methacrolein and for introducing fresh methacrolein into the oxidation stage.